To charge the batteries of battery-powered devices, such as cellular phones, PDAs, remote controls, notebooks etc., or directly power devices such as lamps or kitchen appliances, an inductive power system enabling a wireless power transfer can be applied. Inductive power systems for transferring power or charging mobile devices are, for example, known from WO 2008/050260. Such a system in general comprises a power transmitting device, hereafter called transmitter, comprising a plurality of transmitter coils which can individually be energized, thereby generating an alternating magnetic field. The inductive power system further comprises a power receiving device comprising a load requiring power. In order to receive power, the power receiving device is provided with a receiver coil, in which the alternating magnetic field, provided by the energized transmitter coils, induces a current. This current can drive the load of the receiving device, for example charge a battery or light a lamp. Hereafter a power receiving device is referred to as a receiver comprising a receiver coil and a load.
It is very important to enable a low, preferably (virtual) zero, standby power. For example, when there is no device on the wireless power transmitter, the power dissipation should be almost zero.
US patent application US2008/0157909 provides a system for detecting the coupling between a transmitter coil of the power transmitter device and a receiver coil of the power receiving device. While energizing the transmitter coil, a current sensor monitors the current through the transmitter coil to decide whether the receiver coil of the power receiving device is coupled to the transmitter coil. The system requires the energizing of the transmitter coil on a regular basis and results in too much energy consumption by the power transmitter device especially when no power receiving device is present.
If the system of said US patent application is used in the power transmitter device which comprises a plurality of transmitter coils, each one of the transmitter coils has to be energized on a regular basis, leading to a further increase of power usage. Further, the regular energizing of the transmitter coil introduces at regular instants an electromagnetic field which may cause, for example, electromagnetic interference, or may erase information on a magnetic strip, for example, of a bank card when the bank card is accidentally laid on the power transmitter device.